Digital portable and mobile radios may be arranged either to communicate with one another via base stations, or directly with one another in ‘direct mode’. The communication will typically either be over a digital simplex communication channel, or over a digital semi-duplex communication channel.
FIG. 1 illustrates the general scheme of a personal mobile radio (PMR) system 10. Portable radios 2, 4 and 6 of FIG. 1 can communicate with a base station 8. Radios 2, 4 and 6 could equally well be mobile radios mounted in vehicles. Each of the radios shown in FIG. 1 can communicate through base station 8 with one or more other radios. If radios 2, 4 and 6 are capable of direct mode operation, then they may communicate directly with one another or with other radios, without the communication link passing through base station 8.
Portable and mobile radios may employ a regular payload framing structure. This structure consists of a structure of fixed time points at which parts of the communication begin and end.
Conventionally, in a back-to-back radio system employing a single frequency in a time division duplex fashion to convey duplex voice, each radio utilised the available radio resource for 50% of the time.
However, utilization of the radio resource for 50% of the time does not take advantage of the time discontinuity of human speech, whereby for the majority of the time, only one of the two parties in the call is speaking at any one time. In addition, when a party is speaking, their ability to judge the quality of the audio being conveyed to them is impaired. This is the principle which allows operation of the so-called pseudo-duplex method, where under normal circumstances the majority of the capacity is devoted to the active speaker and then in periods of double-talk (both speakers active) the system reverts to a conventional Time Division Duplex (TDD) scheme. The present application describes an implementation of the controlling protocol for such a pseudo-duplex scheme.
The closest known technologies relate to Voice Activity Detection (VAD) schemes applied to cellular applications of rate control and discontinuous operation and hands-free speaker-phone control techniques. Voice Activity Detection involves measuring the amount of voice signal currently requiring transmission. When the user of a radio is speaking, the VAD measurement is different from when the user is not speaking, and only background noise is detected.
The VAD in cellular systems is biased in order to ensure that when a party speaks, the radio, including the speech codec and RF circuitry etc., will be active to convey that speech to the other party in the presence of background noise and other impairments. However, this leads to transmission of data when a party is not speaking. The cost of this is slightly lower battery life and slightly increased interference to co-channel users in other cells of the system. These are essentially second (or higher) order effects. In these systems, there is no concept of a finite resource being available to the duplex call. It is entirely possible and consistent for the uplink and downlink, which are almost always on different carriers, to be simultaneously utilising the full bandwidth.
In a speaker phone system, in order to overcome the need for acoustic echo control, the phone may be in ‘microphone mode’ or ‘speaker mode’, but never both at the same time. The control algorithm has access to audio signals from the incoming telephone line and the microphone on which to base the decision over which mode to apply at any one time. There is no sense that the speakerphone must convey its decision to another unit since it alone has full control over the decision.
For a pseudo-duplex control scheme, two units must mutually agree on the best channel usage, based upon the activity of the two parties. They must convey their relative VAD measurements to one-another to allow a decision algorithm to be executed jointly. Furthermore, all of this must be done in a controlled manner in the presence of transmission errors which could cause unexpected behaviour.
A need exists to alleviate the problems of the prior art.